1. Field of the Invention
The invention relates to a method for controlling the shifting of an automated twin-clutch transmission including a first transmission unit with a first transmission input shaft, a first engine clutch and a first group of gears and a second transmission unit with a second transmission input shaft, a second engine clutch and a second group of gears, with which a shifting operation is executed between a load gear and a target gear assigned to the same transmission unit by using an intermediate gear assigned to the other transmission unit. The shifting operation is performed as a multiple shifting operation including the following shifting steps: engaging the intermediate gear, performing a clutch change-over from the engine clutch of the load gear to the engine clutch of the intermediate gear, disengaging the load gear, engaging the target gear, performing a clutch change-over from the engine clutch of the intermediate gear to the engine clutch of the target gear, and adjusting the engine speed of the assigned drive engine to reach the synchronous speed of the target gear at the end of the shifting operation.
The configuration of a twin-clutch transmission is for instance disclosed in German Published Non-Prosecuted Patent Application No. DE 35 46 454 A1, which describes a gearshift transmission for an automobile with a twin-clutch. The twin-clutch transmission has a first engine clutch, a first transmission input shaft, and a first group of gears that form a first transmission unit, and a second engine clutch, a second transmission input shaft, and a second group of gears that form a second transmission unit. By engaging one of the gears, it is possible to connect the related transmission input shaft to a common transmission output shaft. In the usual alternate assignment of gears, the even gears are assigned to the first transmission unit and the odd gears are assigned to the second transmission unit. In such an assignment of gears, the shifting operation is simple and sequential from an engaged load gear to the next higher or next lower target gear, i.e. in each case to the target gear assigned to the other transmission unit, as long as the clutch can be locked passively. This sequential shifting operation spans from opening the engine clutch assigned to the transmission unit of the target gear, engaging the target gear, a subsequent overlapped opening of the engine clutch assigned to the transmission unit of the load gear and closing the engine clutch assigned to the transmission unit of the target gear. Thus the power transmission takes place alternately via the first transmission unit with the first engine clutch and the first transmission input shaft and via the second transmission unit with the second engine clutch and the second transmission input shaft. A special advantage of this transmission is that there is no interruption in the traction force and/or the thrust force during the gear change, which is why the twin-clutch transmission is included in the category of powershift transmissions. Since a manual activation of two engine clutches and the gear shifting with two temporarily simultaneously engaged gears would be considerably complex from the mechanical point of view, twin-clutch transmissions are designed to be automated as far as is known. This means, both the activation of the engine clutches and also the gear shifting take place through the use of assigned actuating drives that can be activated electromagnetically, using an electric motor, using a pressure medium, for instance hydraulically or in any other manner.
In certain operating situations that are substantially dependent on a sudden change of the slope of the roadway, for instance a transition from a street extending horizontally to a street rising steeply and/or the performance requirement of the driver, for instance rapidly tromping on the accelerator pedal for rapid acceleration (kick-down), a simple sequential shifting operation cannot sufficiently meet the requirements of the motor control and shift control and/or the requirements of the driver. This case necessitates a shifting operation in which at least one gear is skipped, i.e. the load gear and the target gear are assigned to the same transmission unit. For the purpose of avoiding interruptions in the traction force and/or thrust force even in this case, it is preferred to carry out such a shifting operation as a so-called multiple shifting operation by using an intermediate gear that is assigned to the other transmission unit.
In order to carry out the fastest multiple shifting operation possible, the required speed is adjusted, i.e. the engine speed is conventionally adjusted to the effective synchronous speed of the target gear in the assigned engine clutch during the shifting operation. For this a target speed gradient is predetermined, according to which the engine speed is made to attain the synchronous speed of the target gear at the end of the shifting operation through the use of the motor control and/or the overlap control of the engine clutches. If the actual shifting progress, i.e. the progression in terms of time of the individual shifting steps and the resulting total shifting time correspond to the underlying estimated total shifting time, then the engine speed reaches the synchronous speed of the target gear exactly at the desired point of time. Due to changing operating conditions, particularly different operating temperatures, wear and tear of the engine clutches and of the shifting and synchronization elements of the gear, the partial shifting times of the individual shifting steps and thus even the total shifting time of the multiple shifting operation cannot be determined precisely beforehand. As a result the engine speed mostly does not reach the synchronous speed at the desired point of time. If the shifting operation, i.e. the execution of the individual shifting steps proceeds faster than expected, then the shifting progression hurries ahead of the speed adjustment. This means at the end of the shifting operation a waiting period can be anticipated in which the engine speed still passes through the remaining speed difference for reaching the synchronous speed. If, on the other hand, the shifting operation proceeds slower than expected, then the speed adjustment hurries ahead of the shifting progression. This means toward the end of the shifting operation, the engine speed must remain at the synchronous speed of the target gear that was reached too early until the conclusion of the shifting operation. The first case involves a genuine delay of the entire shifting progression, while in the second case the driver experiences an apparent, possibly unpleasant delay of the shifting progression due to the temporarily constant engine speed.